The present invention relates to a process for producing para-xylene integrating pressure swing adsorption for separating para-xylene and ethylbenzene from mixed C8 aromatics with recovery of para-xylene by crystallization. The present invention further relates to a process comprising using a pressure swing adsorption (PSA) process for separating para-xylene and ethylbenzene from mixed C8 aromatics using a para-selective adsorbent and separating the para-xylene from the para-xylene/ethylbenzene effluent by fractional crystallization to obtain para-xylene product. The para-selective adsorbent is preferably a non-acidic, molecular sieve. The para-selective adsorbent is more preferably a non-acidic, medium pore, molecular sieve. The molecular sieve is preferably of the MFI structure type and the process is preferably operated in the vapor phase at elevated temperatures and pressures wherein the temperature is substantially isothermal. The present invention also relates to a method of pressure swing adsorption which includes a plurality of steps and which provides recovery from a mixture comprising C8 aromatics of a substantially pure para-xylene or para-xylene and ethylbenzene product stream and a substantially pure meta-xylene and ortho-xylene product stream and separating the para-xylene from the para-xylene/ethylbenzene stream by fractional crystallization to obtain a substantially pure para-xylene product.
A pressure swing adsorption process to separate para-xylene and ethylbenzene from C8 aromatics which uses a para-selective, non-acidic, medium pore molecular sieve of the MFI structure type and is operated isothermally in the vapor phase at elevated temperatures and pressures is used to produce a para-xylene-rich stream which is then subjected to crystallization to produce a para-xylene product. In the pressure swing adsorption step, a fixed bed of adsorbent is saturated with pX and EB, which are preferentially adsorbed, then the feed to the process is stopped. Lowering the partial pressure desorbs the pX and EB. The process effluent is rich in pX and EB. A stream of non-adsorbed mX and oX may be obtained prior to desorption of pX and EB. The mX/oX stream may be isomerized to produce an equilibrium mixture of xylenes and recycled to the PSA step.
It is known that certain high surface area, porous substances such as silica gel, activated charcoal, and molecular sieves, including zeolites and other molecular sieves, have certain selective adsorption characteristics useful in separating a hydrocarbon mixture into its component parts.
The selective sorption properties of molecular sieves and zeolites have been disclosed in earlier patents and in literature references. Crystalline molecular sieves and zeolites are shape-selective in that they will admit molecules of specific geometry while excluding other molecules.
The separation of xylene isomers has been of particular interest because of the usefulness of para-xylene in the manufacture of terephthalic acid which is used in the manufacture of polyester fabric. Other components of the C8 aromatic hydrocarbon feedstream from which para-xylene (pX) is generally produced are ortho-xylene (oX), which is used in the manufacture of phthalic anhydride which is used to make phthalate based plasticizers; meta-xylene (mX), which is used in the manufacture of isophthalic acid used in the production of specialty polyester fibers, paints, and resins; and ethylbenzene (EB) which is used in the manufacture of styrene.
A refinery feedstock of aromatic C8 mixtures containing ethylbenzene and xylenes will typically have the following content:
Equilibrium mixtures of C8 aromatic hydrocarbons generally contain about 22 weight percent para-xylene, about 21 weight percent ortho-xylene, and about 48 weight percent meta-xylene in the equilibrium mixture.
Processes to separate xylene isomers include low temperature crystallization, fractional distillation, selective sulfonation with subsequent hydrolysis and selective solvent separation; however, such processes require high operating costs.
The use of faujasite zeolites, which are large pore type X and Y type zeolites, as adsorbents in liquid phase, chromatographic-type separations is well known.
In the petrochemical production chain, one of the most important streams is the C6 to C8 aromatics stream containing benzene, toluene, and xylenes (BTX), which is a source of raw materials for high value downstream products. Of the C8 aromatics, para-xylene (pX) is the most desirable. However, because the boiling points of ethylbenzene (EB), ortho-xylene (oX), meta-xylene (mX) and para-xylene (collectively referred to as xe2x80x9cC8 aromaticsxe2x80x9d) are close, they are difficult to separate by fractional distillation. As a consequence, various alternative methods of separating pX from the C8 aromatics have been developed. Common separation methods are fractional crystallization, which utilizes the difference in freezing points, and liquid phase adsorption (e.g., UOP""s Parex process and IFP""s Eluxyl process), which uses a faujasite zeolite to chromatographically separate pX from the other C8 aromatics. The reject stream from the crystallization process or the raffinate from the adsorption process are depleted in pX, and contain relatively high proportions of EB, oX and mX. These streams are typically sent to a catalyst reactor, where the xylenes are isomerized to equilibrium, and at least a portion of the EB is converted to other products, which can be removed from the C8 aromatics by fractional distillation.
Processes for making pX have typically included combinations of isomerization with fractional crystallization or adsorption separation. FIG. 1 is a schematic representation of known art combination of an isomerization catalyst reactor and a crystallization unit. Crystallization is a separation process that takes advantage of the fact that pX crystallizes before the other isomers, i.e., pX crystallizes at 13.3xc2x0 C. (55.9xc2x0 F.), whereas oX crystallizes at xe2x88x9225.2xc2x0 C. (13.4xc2x0 F.) and mX at xe2x88x9247.9xc2x0 C. (xe2x88x9254.2xc2x0 F.). In the physical system of the three isomers, there are two binary eutectics of importance, the pX/mX and the pX/oX. As pX is crystallized from the mixture, the remaining mixture (mother liquor) composition approaches one of these eutectic binaries, depending on the starting composition of the mixture. Therefore, in commercial practice, pX is crystallized so that the binary eutectic is only approached but not reached to avoid co-crystallization of the xylene isomers, which would lower the pX purity. Thus, the key disadvantage for crystallization is restricted pX recovery per pass, due to this eutectic limit of the C8 stream. Typically, the concentration of pX in a mixed C8 aromatic stream at equilibrium is about 22 wt %. In commercial crystallization operations, the eutectic point of this mixture limits the pX removed per pass to about 65% of that amount.
The problem of the eutectic limit for pX crystallization has been recognized for some time. U.S. Pat. No. 5,329,060 discloses that the eutectic point of the crystallization unit can be overcome by use of a selective adsorption zone that enriches the pX feed to the crystallizer by rejecting most of the mX, oX and EB to the isomerization reactor. Specifically, the disclosure teaches using a faujasite-based, liquid phase adsorption process that can either be selective for pX or selective for mX and oX. The result of this process is a stream enriched in pX, but still containing a substantial portion of mX and oX. Similarly, U.S. Pat. No. 5,922,924 discloses combining at least one liquid phase, simulated moving bed adsorption zone with crystallization to produce high purity pX. Again, pX is enriched, but the stream still contains significant mX and oX.
U.S. Pat. No. 3,699,182 discloses use of zeolite ZSM-5 in a process for selective separation of biphenyls from mixtures containing the same and para-disubstituted aromatic isomers from mixtures containing the same, particularly for separating C8 aromatics using ZSM-5 zeolite.
U.S. Pat. No. 3,724,170 discloses chromatographic separation of C8 aromatic mixtures over zeolite ZSM-5 or ZSM-8, which has preferably been reacted with an organic radical-substituted silane, in at least two distinct stages whereby para-xylene and ethylbenzene are selectively absorbed whereas the meta-xylene and ortho-xylene are not adsorbed, removing the unadsorbed meta-xylene and ortho-xylene, eluting the para-xylene followed by the ethylbenzene.
British Pat. No. 1,420,796 discloses use of zeolite ZSM-5 or ZSM-8, preferably ZSM-5 or ZSM-8 zeolites which have been reacted with certain silanes, for adsorptive separation of para-xylene and ethylbenzene from a mixture of para-xylene, ortho-xylene, meta-xylene, and ethylbenzene by adsorption/desorption using two or more columns operated in a parallel manner so that when adsorption is being conducted in one column, desorption can be conducted in a parallel column under such conditions as to obtain a continuously operating process which is said to have faster results than use of a single column alone. It is stated that 250xc2x0 C. (482xc2x0 F.) is a preferred upper limit as operation above 250xc2x0 C. (482xc2x0 F.) may lead to catalytic conversion in the zeolite-containing column.
U.S. Pat. No. 3,729,523 discloses a process for separating and recovering each of the xylene isomers and ethylbenzene wherein a mixture of C8 aromatic hydrocarbons, which may also contain C9 and higher paraffins, is heated to 50xc2x0-500xc2x0 F. (10xc2x0-260xc2x0 C.) and subjected to an adsorption step to recover a first mixture of para-xylene and ethylbenzene and a second mixture comprising meta-xylene, ortho-xylene, and the C9 and higher aromatics. The adsorption is preferably conducted in the presence of a molecular sieve or synthetic crystalline aluminosilicate zeolite as the adsorbent, with ZSM-5, the preferred zeolite. The para-xylene and ethylbenzene are adsorbed and may be recovered by heating the adsorbent, reducing the partial pressure of the sorbed material in the vapor or liquid surrounding the adsorbent, lowering the total pressure of the system or purging with a suitable inert gas or displacement liquid. The resulting para-xylene and ethylbenzene mixture is then subjected to crystallization to recover para-xylene and the mother liquor is subjected to distillation to recover the ethylbenzene.
Chinese Patent Application No. 1136549 discloses selectively adsorbing pX and EB from a C8 isomer stream using silicalite-1 zeolite and then producing  greater than 99.5% purity mX and oX from the portion of the stream not adsorbed. In this process there is a substantial amount of contaminating feedstream in the voids of the silicalite-1 adsorbent which is not removed and comes off the adsorption bed along with the adsorbed pX and EB so that the desorbed stream is not substantially pure pX and EB but contains significant amounts of unseparated oX and mX.
U.S. Pat. No. 6,111,161 discloses a process for the production of high purity para-xylene from a charge containing C7-9 aromatic hydrocarbons in which a first fraction is enriched to at least 30% weight with para-xylene and this fraction is purified by at least one high-temperature crystallization in at least one crystallization zone. Said first fraction is crystallized in a crystallization zone at high temperature T1 and advantageously between +10 and xe2x88x9225xc2x0 C., crystals in suspension in a mother liquor are recovered, the crystals are separated from the mother liquor in at least a first separation zone, the crystals obtained are partially melted in at least a zone for partial melting and a suspension of crystals is recovered, the crystals in suspension are separated and washed in at least one separation and washing zone and pure para-xylene crystals and washing liquor are recovered, and pure crystals are optionally completely melted and a liquid stream of melted para-xylene is collected.
U.S. Pat. No. 5,448,005 discloses a process for producing high purity para-xylene from a high weight percent para-xylene feedstock, comprising at least about 70 wt % para-xylene and preferably at least about 80 wt % para-xylene which uses a single temperature crystallization production stage at a temperature in the range of from about 0xc2x0 F. to about 50xc2x0 F. and also uses scavenger stages to raise the para-xylene recovery rate. The single temperature production stage crystallizer of the process employs a wash using only para-xylene product.
It is generally recognized by those skilled in the art that using crystallization for separating and purifying pX from a mixture of C8 aromatics suffers from the major disadvantage that the maximum recovery level per pass is limited to approximately 60-65% due to the existence of eutectics between pX and the other C8 aromatic hydrocarbons, and that it is desirable to increase the para-xylene content of the stream going to the crystallizer above that typical of mixed xylenes (i.e., 22-24%).
For example, U.S. Pat. No. 5,284,992 describes increasing the para-xylene content of the stream going to the crystallizer by first separating the C8 aromatic stream via simulated moving bed adsorption chromatography, which produces a stream containing 75-98% pX and a second stream containing a mixture of mX, oX and ethylbenzene.
U.S. Pat. No. 5,329,060 teaches combining crystallization with a similar chromatographic separation using Faujasite aluminosilicates or the closely related type X and type Y aluminosilicates, which are non para-selective molecular sieves.
U.S. Pat. No. 3,729,523 discloses a process for recovering high purity streams of all the C8 aromatic xylene isomers including ethylbenzene by combining selective adsorption and crystallization.
Continuous liquid chromatography methods of separation (e.g., simulated countercurrent like UOP""s PAREX) are complex and use a solvent (i.e., liquid desorbent) to remove the adsorbed phase. This results in the necessity of an additional column for separating by fractional distillation the desorbent for reuse. Fixed bed temperature (thermal) swing adsorption/desorption is limited by the long times (hours to days) necessary to increase or decrease the temperature of the adsorbent bed.
None of the prior art describes a pressure swing adsorption process for separating pX from a C8 aromatic mixture. Pressure swing adsorption offers the advantage of reduced complexity, no liquid desorbent and opportunities for better synergy with the rest of the para-xylene unit (energy savings), e.g., directing the non-adsorbed phase (mX and oX) exiting the adsorption unit at high temperature directly to the xylene isomerization reactor.
None of these references discloses a process using pressure swing adsorption employing a para-selective adsorbent which is preferably a large crystal, non-acidic medium pore molecular sieve in combination with fractional crystallization and, optionally, isomerization, to obtain high purity para-xylene in high yield.
Molecular sieves are crystalline oxides having pore openings and internal cavities the size of some molecules. Zeolites, a sub-group of molecular sieves, are crystalline aluminosilicates. Another well known sub-group of molecular sieves are aluminophosphates or ALPOs. In general, molecular sieves are classified into three groups based on pore size: small pore molecular sieves with pore diameters from 3-4 xc3x85; medium pore molecular sieves with pores diameters from 4-6 xc3x85; and large pore molecular sieves with pore openings of 6-8 xc3x85. In addition to the molecular size pores, molecular sieves have high adsorption energies and for many years have been used as adsorbents. By selection of the proper pore size, molecular sieves may selectively adsorb molecules of different size. This molecular sieving leads to adsorption and separation of the smaller molecule. Often molecular sieving selectivities are high, 100 or greater. The separation of branched from linear paraffins is a commercial process, which utilizes the small pore A zeolite.
Large pore molecular sieves have also been utilized in the separation of hydrocarbons. In large pore molecular sieves, however, all components diffuse into the pores and the separation is based on differences in adsorption energies. The molecule with the highest bond energy is preferentially adsorbed. Generally, adsorption selectivities are high only when molecules have very different heats of adsorption, for example water and paraffin. For molecules with similar heats of adsorption, the adsorption selectivities are low, ca. 1-4. Xylenes isomers, for example, have similar heats of adsorption in Y zeolite. Due to small differences in heats of adsorption and packing geometry in BaY, pX displays an adsorption selectivity of about 2 compared with the other C8 aromatics. In order to separate pX in sufficient purity for chemical sale, i.e., greater than 99%, many separation stages must be conducted. This type of process operates on principles similar to that of chromatography. Commercial examples of separations of this type are the UOP Parex and IFP Eluxyl liquid phase adsorption processes, which utilize ion exchanged Y zeolites to separate pX from C8 aromatics.
Adsorbents useful in the present invention are based on molecular sieves that selectively adsorb p-xylene within the channels and pores of the molecular sieve while not effectively adsorbing m-xylene and o-xylene C8 isomers (i.e., total exclusion of the larger m-xylene and o-xylene or having much slower adsorption rates compared to p-xylene.).
Molecular sieves are ordered porous crystalline materials, typically formed from silica, alumina, and phosphorus oxide (PO4) tetrahedra, that contain a crystalline structure with cavities interconnected by channels. The cavities and channels within the crystalline structure are uniform in size and may permit selective separation of hydrocarbons based upon molecular dimensions. Generally, the term xe2x80x9cmolecular sievexe2x80x9d includes a wide variety of natural and synthetic crystalline porous materials which typically are based on silica tetrahedra in combination with other tetrahedral oxide materials such as aluminum, boron, titanium, iron, gallium, and the like. In these structures networks of silicon and elements such as aluminum are cross-linked through sharing of oxygen atoms. Substitution of elements such as aluminum or boron for silicon in the molecular sieve structure produces a negative framework charge which must be balanced with positive ions such as alkali metal, alkaline earth metal, ammonium or hydrogen. Molecular sieve structures also may be formed based on phosphates in combination with other tetrahedrally substituted elements such as aluminum.
Adsorbents useful in this invention should not possess catalytic isomerization or conversion activity with respect to the C8 aromatic feedstream. Thus, suitable molecular sieves should be non-acidic. If an element such as aluminum or gallium is substituted in the molecular sieve framework, the sieve should be exchanged with a non-acidic counter-ion, such as sodium, to create a non-acidic sieve adsorbent.
Examples of molecular sieves suitable as adsorbents useful in this invention include zeolitic materials containing pore dimensions in the range of 5 to 6 xc3x85 (10xe2x88x928 meter), typically 5.1 to 5.7 xc3x85, and preferably 5.3 to 5.6 xc3x85, as measured in cross axes of the pore. This range typically is referred to as xe2x80x9cmedium porexe2x80x9d and typically contains 10-ring tetrahedra structures. Typical examples of medium pore molecular sieves include those with MFI and MEL framework structures as classified in Meier and Olson, xe2x80x9cAtlas of Zeolite Structure Types,xe2x80x9d International Zeolite Association (1987), incorporated herein by reference in its entirety. A small pore molecular sieve, such as A zeolite, which contains 8-ring structures does not have a sufficiently large pore opening to effectively adsorb para-xylene within the sieve. Most large pore molecular sieves, such as mordenite, Beta, LTL, or Y zeolite, that contain 12-ring structures do not adsorb para-xylene selectively with respect to ortho- and meta-xylenes. However, several 12 ring structures, having a smaller effective pore size, for example due to puckering, are potentially useful in the invention, such as structure types MTW (e.g., ZSM-12) and ATO (e.g., ALPO-31).
Specific examples of molecular sieves include ZSM-5 (MFI structure type) and ZSM-11 (MEL structure type) and related isotypic structures. Since suitable adsorbents should not be catalytically reactive to components in the feedstream, the preferable adsorbent useful in this invention is silicalite (MFI structure type), an essentially all silica molecular sieve, which contains minimal amounts of aluminum or other substituted elements. Typically, the silica/alumina ratio of suitable silicalite is above 200 and may range above 1000 depending on the contaminant level of aluminum used in the sieve""s preparation. Other MFI and MEL sieves may be use to the extent they are made non-catalytically active. MFI-based molecular sieves are preferred in this invention with silicalite as the most preferred. Other potentially useful adsorbents include structure types MTU, FER EUO, MFS, TON, AEL, ATO, NES, and others with similar pore sizes.
A molecular sieve which is not catalytically reactive will typically exhibit less than 10% conversion of pX to mX and oX, and preferably less than 5%, and most preferably less than 1%, at the temperature of operation for the process of the invention.
Attempts have been made to use adsorption with zeolites such as ZSM-5 and ZSM-8 to separate ethylbenzene (EB), para-xylene (pX), meta-xylene (mX), and ortho-xylene (oX) from mixtures of C8 aromatics; however, a major disadvantage of these processes is that the time required to effect desorption of the adsorbed components is too long to provide a commercially useful process. In addition, with acidic zeolites, such as HZSM-5, the high temperatures used to obtain rapid desorption cause catalytic reactions to occur converting pX to mX and oX and converting EB to benzene. Furthermore, with HZSM-5, traces of olefins, which are usually present in commercial feeds, irreversibly chemisorb lowering the adsorption capacity of the zeolite. As a result, frequent reconditioning of the adsorbent (e.g., removal of coke deposits) is required.
Due to the strong adsorption and reactivity of xylenes on acid sites of adsorbents such as HZSM-5, a commercial separation process has not been developed. We describe the use of silicalite in a high temperature process to effect the separation of para-xylene and ethylbenzene from a C8 aromatic mixture without reaction of the adsorbed hydrocarbons. These adsorbent and process modifications solve the previous technical obstacles, which have limited commercial development of a molecular sieving, selective adsorption/desorption process for separation of C8 aromatic hydrocarbons.
The process of the present invention overcomes disadvantages of known processes by using pressure swing adsorption at elevated temperature and pressure with a non-acidic, molecular sieve-containing adsorbent to accomplish a rapid adsorption and desorption of the desired components from a feedstream containing C8 aromatics and provide a rapid separation of the desired components which is suitable for commercial use. A non-acidic molecular sieve, such as silicalite (MFI structure type with little to no aluminum), is used to selectively adsorb pX and EB. Desorption is significantly faster and reactions of the adsorbed molecules (pX and EB) do not occur. In addition, olefins do not adsorb on the silicalite, so the adsorption capacity of the adsorbent remains high and frequent reconditioning is not required.
The effluent stream of pX and EB obtained following pressure swing adsorption is has a higher concentration of pX than is typically obtained with isomerization, and, in optimum conditions, may be substantially free of mX and oX. When separation of C8 aromatics by pressure swing adsorption is combined with crystallization, this has the advantage of enabling one to increase pX production capacity. Due to the higher concentrations of pX obtained with PSA, it will also be possible to reduce costs of purifying pX by conducting the crystallization of the para-xylene-containing product stream at higher temperatures, if desired. This would reduce energy consumption and may eliminate the need for extremely low-temperature ethylene-cooled crystallization facilities which would be a further cost savings.
The disclosed invention provides a novel and improved process for producing para-xylene employing a pressure swing adsorption unit containing an adsorbent selective for para-xylene and ethylbenzene in conjunction with crystallization technology for separating and recovering para-xylene to improve the overall yield and per pass recovery of para-xylene. The process of the present invention will allow the capacity of existing crystallization units to be increased at minimal capital investment and will also benefit new units, by reducing capital and utility and raw material costs.
Two commercial processes are practiced for separating para-xylene (pX) from the other C8 aromatic isomers, fractional crystallization and liquid phase adsorption. The advantage of crystallization is high product purity; however, the key disadvantage is restricted pX recovery per pass, due to the eutectic limit of the C8 stream. Typically, the concentration of pX in a mixed C8 aromatic stream at equilibrium is about 22 wt %. In commercial crystallization operations, the eutectic point of this mixture limits the pX removed per pass to about 65%. The process of the present invention increases the efficiency of the crystallizer for removing pX by first passing the mixed C8 aromatic stream through a pressure swing adsorption (PSA) unit to remove a significant portion of the meta-xylene (mX) and ortho-xylene (oX) in the stream. This increases the pX concentration in the crystallizer feed and lowers the temperature at which pX can be crystallized without co-crystallizing impurities, resulting in significantly more pX removed per pass. Furthermore, the pX-depleted stream gives lower xylene loss when isomerized in an isomerization reactor, which increases the overall yield of pX for the unit. Recovery of pX can be further enhanced by pretreatment of the feed to the PSA unit and/or to the isomerization reactor to reduce the concentration of EB in the feed. This can be accomplished by contacting the feed with an ethylbenzene conversion catalyst.
An additional catalyst reactor may be used to pretreat the C8 aromatic feed to convert at least a portion of the ethylbenzene to xylenes or products which can be separated by fractional distillation from the C8 aromatics prior to sending the C8 aromatic feedstream to the PSA unit.
In one embodiment of the invention, an additional catalyst reactor may be used to treat the para-xylene-lean reject stream from the separation by crystallization of pX from the pX/EB stream from the PSA unit to convert at least a portion of the ethylbenzene in it to xylenes or products which can be separated by fractional distillation from the C8 aromatics prior to sending the C8 aromatic feedstream to the PSA unit.
The para-xylene production unit, in addition to a PSA unit (and, optionally, a separation/purification unit for separating pX from pX/EB) used in the process of the present invention may also contain a catalyst reactor for isomerization of aromatics and one or more distillation columns for separation of aromatics as well as a catalyst reactor for pretreatment of a C8 aromatic feed to reduce the amount of ethylbenzene in the feed by ethylbenzene conversion.
The catalyst system in the additional catalyst reactor used to convert ethylbenzene can be any catalyst system suitable for ethylbenzene dealkylation, hydrodeethylation or hydroisomerization. Examples of catalyst systems for dealkylation are disclosed in Re. 31,782 and U.S. Pat. No. 4,908,342, incorporated herein by reference in their entireties. Examples of catalyst systems for hydrodeethylation are disclosed in U.S. Pat. No. 4,899,011 and U.S. Pat. No. 5,367,099 incorporated herein by reference in their entireties. Examples of catalyst systems for hydroisomerization are disclosed in U.S. Pat. Nos. 5,028,573, 6,150,292 and 5,908,967 incorporated herein by reference in their entireties.
Many of the chemical and physical properties of xylene isomers and ethylbenzene are very similar making separation difficult. The molecular size of these isomers, however, is slightly different and is determined by the position of methyl substitution. The kinetic diameter of para-xylene and ethylbenzene are approximately 6.0 xc3x85; whereas meta-xylene and ortho-xylene are slightly larger, 6.8 xc3x85. It has been known for many years that, based on these differences in size, medium pore zeolites, such as HZSM-5, can selectively adsorb para-xylene and ethylbenzene [See U.S. Pat. Nos. 3,653,184; 3,656,278; 3,770,841; 3,960,520; 4,453,029; 4,899,017; Wu, et al. STUD. SURF. SCI. CATAL., 28:547(1996); Yan, T. Y., IND. ENG. CHEM. RES. 28:572(1989); and Choudhary, et al., IND. ENG. CHEM. RES. 36:1812(1997)] However, a disadvantage of using HZSM-5 for such separations is that protonation of the aromatic ring by acid sites in ZSM-5 leads to formation of a strong chemical bond [Farneth, et al., LANGMUIR, 4:152(1988)] resulting in low desorption rates and long desorption times at low temperature. As a result, such excessively large amounts of ZSM-5 would be required for commercial scale separation of para-xylene and ethylbenzene under these conditions that such separations are not commercially feasible. Increasing the desorption temperature does increase the desorption rate, which lowers the amount of adsorbent needed; however, the acid sites on the HZSM-5 zeolite also have catalytic properties which cause undesirable isomerization of para-xylene to meta-xylene and ortho-xylene, significantly reducing para-xylene purity. Another disadvantage is that the acid sites strongly adsorb olefins which are typically present along with the C8 aromatics in the feedstream, thus lowering the capacity of the adsorbent to adsorb para-xylene and ethylbenzene. These olefins can only be desorbed at high temperatures. Thus, there is either a loss of adsorption capacity at low temperature or a loss in selectivity at high temperature due to reactions catalyzed by the acid sites.
Disadvantages of the earlier processes are overcome in the present invention by using a pressure swing adsorption process for separating para-xylene and ethylbenzene from mixed C8 aromatics using a non-acidic, medium pore molecular sieve, preferably of the MFI structure type and preferably operating in the vapor phase at elevated temperatures and pressures followed by separation and purification of the para-xylene by crystallization. The present invention uses a pressure swing adsorption process which includes a plurality of steps and which provides recovery from a mixture comprising C8 aromatics of a para-xylene or para-xylene and ethylbenzene product stream containing higher concentrations of para-xylene or para-xylene and ethylbenzene than are obtainable by the typical isomerization processes as well as a meta-xylene and ortho-xylene product stream having enhanced amounts of mX and oX. The para-xylene is then separated from the para-xylene/ethylbenzene stream by fractional crystallization to obtain a substantially pure para-xylene product. An advantage of the process is that the PSA unit does an initial bulk separation of pX/EB from mX/oX prior to product purification by crystallization. In the process of the invention, mX and oX no longer go to the crystallization unit but are sent back to the catalyst section. The composition of the pX/EB stream comprises at least 50 mole percent pX/EB. Thus, with the present invention, more pX is recovered per pass and more pX can be produced.
We have found that non-acidic forms of ZSM-5, such as Na-ZSM-5, are preferred adsorbents over HZSM-5. In particular, silicalite is a preferred adsorbent over HZSM-5. Silicalite, an all silica, isostructural form of ZSM-5 has been shown to possess superior properties. Like ZSM-5, silicalite selectively adsorbs pX and EB; however, desorption is significantly faster, since the molecules are only adsorbed physically not chemically, as with HZSM-5. Moreover, pX does not isomerize, even at the elevated temperatures necessary to make the process economically practicable.
In silicalite, a silica analog of H-ZSM-5, pX and EB are selectively adsorbed due to their smaller size. However, unlike H-ZSM-5, silicalite contains no acid sites. As a result, pX and EB are desorbed at high temperature without reaction. At elevated temperature, the desorption rates are high and the cycle times are much shorter. As a result, much less adsorbent is required. Furthermore, the adsorption capacity does not decrease significantly with repeated adsorption/desorption cycles due to adsorption of olefins in the aromatic stream.
The present invention uses selective adsorption (adsorption of the smaller C8 isomers) and selective desorption (i.e., no isomerization upon desorption) at substantially isothermal temperatures to provide a substantially pure product stream of para-xylene and ethylbenzene and a substantially pure stream of ortho-xylene and meta-xylene. The components in these streams can be further separated to provide substantially pure para-xylene, ethylbenzene, ortho-xylene, and meta-xylene products.
The problems of long desorption times or the need for excessively large amounts of adsorbent have made earlier attempts to separate C8 aromatics by molecular sieving commercially impracticable. In addition to these disadvantages, there is also the problem of how to remove C8 aromatic feed that collects in non-selective voids, that is, feed which collects in the non-selective void volume (i.e., large mesopores in the adsorbent, interstitial space between adsorbent particles, and void space in the adsorbent vessel) so that the purity of the desorbed product stream will not be reduced by this material. The art has not recognized how to overcome this problem for C8 aromatics.
The present invention has solved this problem by selectively separating the C8 aromatic feed that is contained in the non-selective void volume so that a high purity stream of para-xylene and ethylbenzene is obtained following desorption. A high purity stream of mX and oX is also obtained by the process of the invention. In one embodiment of the invention this high purity stream of mX/oX is obtained by separating the mX/oX from the non-selective void volume prior to desorbing the pX/EB.
The use of the process of the present invention in para-xylene production facilities would significantly reduce the amount of meta-xylene and ortho-xylene sent to a crystallization section, thus opening up capacity and decreasing operating costs. This would increase the para-xylene concentration and yields. Having a stream with a greater concentration of para-xylene going to the crystallization section may also make it possible to eliminate a crystallizer. For example, a low-temperature ethylene unit might not be needed if a feed with a higher concentration of para-xylene is being crystallized to recover para-xylene. This would also save equipment costs and reduce the amount of energy necessary to conduct the crystallization and purification of para-xylene.
We have determined that PSA does not need to produce product quality pX in order to provide an advantage over existing technology. Rather, the PSA unit offers substantial benefits when used to concentrate pX in existing or new plants using crystallization for pX recovery. The process of the invention described in this disclosure comprises a pressure swing adsorption unit, and a crystallization section, wherein, the efficiency of the crystallizer for removing pX is improved by first passing the mixed C8 aromatic stream through a PSA unit to remove a portion of the mX and oX in the stream, such that the pX-rich stream contains at least 50 mole % pX and EB relative to the other C8 aromatics. This increases the pX concentration in the crystallizer feed and lowers the temperature at which pX can be crystallized without co-crystallizing impurities, resulting in significantly more pX removed per pass. Furthermore, the pX-depleted stream from the PSA unit, if recycled to an isomerizer, gives lower xylene loss in the isomerization reactor, which increases the overall yield of pX for the unit. The para-xylene production unit, in addition to a PSA unit and a crystallization section used in the process of the present invention may also contain a catalyst reactor for isomerization of aromatics and one or more distillation columns for separation of aromatics as well as a catalyst reactor for pretreatment of a C8 aromatic feed to reduce the amount of ethylbenzene in the feed by ethylbenzene conversion.
The present invention is therefore directed to a chemical plant and process which offers an improvement over the prior art for the recovery of pX from streams which comprise C8 aromatics. The present invention resides in the specific application of a pressure swing adsorption unit and process in conjunction with a fractional crystallization process. This invention utilizes a molecular sieve adsorbent which is selective for pX and EB, and rejects mX and oX to produce an effluent stream comprising pX/EB at a concentration of 50 mole % or greater(relative to the total C8 aromatics).
In a preferred embodiment of the invention, the crystallization process used in combination with PSA has an advantage over other crystallization processes. It reduces the refrigeration requirements compared to designs disclosed in U.S. Pat. Nos. 6,111,161 and 5,448,005. Thus it requires less energy expenditure and provides a cost savings. It accomplishes this by separating some or most of the final product early in the separation sequence thereby reducing the amount of material that requires lower temperature refrigeration. It does not recycle cake back to the first crystallizer from the lower temperature stage(s), but rather uses a reslurry drum to sufficiently warm the crystals so that additional para-xylene product can be recovered without the need for more refrigeration. As calculated according to standard engineering practices, the refrigeration compressor horsepower for the invention can be as much as 13% less than that for comparable designs based on the teachings of U.S. Pat. No. 6,111,161. Combining the advantages of this crystallization process, in which pX in a feed containing higher concentrations of pX can be separated at higher temperatures, without the need for additional processing, with the advantages obtained by using PSA for separation of pX or pX and EB from a C8 aromatic feed increases the overall advantages of the process of the present invention over known para-xylene production processes.
The present invention relates to a process for producing high purity para-xylene from mixed C8 aromatics comprising subjecting the C8 aromatic feed to a pressure swing adsorption (PSA) process to separate para-xylene or para-xylene and ethylbenzene from a mixture containing C8 aromatics and then subjecting the stream of para-xylene or para-xylene and ethylbenzene recovered to crystallization to obtain high purity para-xylene.
The present invention also relates to a process for the production of substantially pure para-xylene from a feedstream comprising C8 aromatics which contains para-xylene, meta-xylene, ortho-xylene, and ethylbenzene, said process comprising introducing a feedstream comprising C8 aromatics into a pressure swing adsorption unit to produce a stream comprising para-xylene or para-xylene and EB and having no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics in which the pX concentration is greater than that found in an equilibrium mixture of xylene isomers and a stream enriched in mX and oX having no more than a total of about 25 mole percent of para-xylene based on total C8 aromatics; and passing at least a portion of the para-xylene-enriched stream to a crystallization unit to produce a high purity para-xylene product stream and a para-xylene-lean stream.
A para-xylene-lean reject stream from the crystallization unit which comprises C8 aromatics may be sent to a catalyst reactor, where the xylenes are isomerized to equilibrium and where at least a portion of any ethylbenzene in the stream is converted to products which can be separated by fractional distillation from the C8 aromatics. The para-xylene-lean reject stream may be combined with the mX/oX-rich effluent stream from the PSA prior to sending it to the isomerization reactor. An additional catalyst reactor may be used to pretreat the C8 aromatic feed to convert at least a portion of the ethylbenzene to xylenes or products which can be separated by fractional distillation from the C8 aromatics prior to sending the C8 aromatic feedstream to the PSA unit. An additional catalyst reactor may be used to treat the para-xylene-lean reject stream from the crystallization unit to convert at least a portion of any ethylbenzene in the stream to xylenes or products which can be separated by fractional distillation from the C8 aromatics prior to sending the C8 aromatics to the PSA unit.
The PSA component of the present invention also relates to a pressure swing adsorption (PSA) process for separating para-xylene, or para-xylene and ethylbenzene, from mixed C8 aromatics using an adsorbent comprising a para-selective adsorbent. The adsorbent is preferably a para-selective, non-acidic, molecular sieve. The adsorbent is more preferably a para-selective, non-acidic, medium pore molecular sieve. The para-selective, non-acidic medium pore molecular sieve is preferably selected from the group of molecular sieve structure types consisting of MFI, TON, MTT, EUO, MEL, and FER. The molecular sieve is preferably of the MFI structure type and the process is preferably operated in the vapor phase at elevated temperatures and pressures wherein the temperature is substantially isothermal. The para-xylene which has been separated from a C8 aromatic mixture using the above PSA process is then purified by crystallization.
The PSA component of the present invention also relates to a pressure swing adsorption process for separating para-xylene from a feed comprising a gaseous mixture comprising meta-xylene and ortho-xylene under substantially isothermal conditions comprising:
(a) adsorbing the mixture onto an adsorbent comprising a para-selective adsorbent capable of selectively adsorbing para-xylene at a temperature and pressure at which at least 0.01 grams of para-xylene may be adsorbed per gram of adsorbent;
(b) producing a first effluent stream having an enriched concentration of ortho-xylene and meta-xylene;
(c) selectively removing feed from non-selective voids;
(d) selectively desorbing para-xylene by decreasing partial pressure of para-xylene; and
(e) collecting a stream having an enriched concentration of para-xylene.
The present invention additionally relates to a pressure swing adsorption process for separating para-xylene from a feed comprising a gaseous mixture comprising para-xylene, meta-xylene, ortho-xylene, and ethylbenzene under substantially isothermal conditions comprising:
(a) adsorbing the mixture onto an adsorbent comprising a para-selective adsorbent capable of selectively adsorbing para-xylene and ethylbenzene at a temperature and pressure at which at least 0.01 grams of para-xylene may be adsorbed per gram of adsorbent;
(b) producing a first effluent stream having an enriched concentration of ortho-xylene and meta-xylene;
(c) selectively removing feed from non-selective voids;
(d) selectively desorbing para-xylene by decreasing partial pressure of para-xylene; and
(e) collecting a stream having an enriched concentration of para-xylene and ethylbenzene.
The PSA component of the present invention further relates to a process for separating and recovering para-xylene from a gaseous mixture comprising C8 aromatic hydrocarbons, the process comprising:
(a) introducing a gaseous mixture comprising meta-xylene, ortho-xylene, and para-xylene into a pressure swing adsorption unit and subjecting the mixture to pressure swing adsorption under substantially isothermal conditions using an adsorbent comprising a para-selective adsorbent capable of selectively adsorbing para-xylene at a temperature and pressure at which at least 0.01 grams of para-xylene and ethylbenzene may be adsorbed per gram of adsorbent to produce a meta-xylene and ortho-xylene-rich effluent stream comprising a mixture of ortho-xylene and meta-xylene, which contains no more than a total of about 20 mole percent of para-xylene based on total C8 aromatics, and a para-xylene-rich effluent stream which contains no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics;
(b) sending at least a portion of the para-xylene-rich stream to a crystallization unit and crystallizing said para-xylene-rich stream to produce a para-xylene product stream and a para-xylene-lean mother liquor stream;
(c) sending at least a portion of the meta-xylene and ortho-xylene-rich stream to an isomerization unit and isomerizing said stream to produce an isomerizate comprising an equilibrium mixture of xylenes;
(d) recycling at least a portion of the isomerizate from step (c) to step (a);
(e) sending at least a portion of the para-xylene-lean mother liquor stream from step (b) to an isomerization unit and isomerizing said stream to produce an isomerizate comprising an equilibrium mixture of xylenes; and
(f) recycling at least a portion of the isomerizate from step (e) to step (a).
In the PSA process component of the invention, a stream having an enriched concentration of para-xylene will contain a greater concentration of para-xylene than the C8 aromatic feedstream from which it was separated by PSA, and a stream having an enriched concentration of ortho-xylene and meta-xylene will contain a greater concentration of ortho-xylene and meta-xylene than the C8 aromatic feedstream from which it was separated by PSA, and a stream having an enriched concentration of para-xylene and ethylbenzene will contain a greater concentration of para-xylene and ethylbenzene than the C8 aromatic feedstream from which it was separated by PSA.
The PSA component of the present invention also relates to a process for separating and recovering para-xylene from a gaseous mixture comprising C8 aromatic hydrocarbons, the process comprising:
(a) introducing a gaseous mixture comprising meta-xylene, ortho-xylene, para-xylene, and ethylbenzene into a pressure swing adsorption unit and subjecting the mixture to pressure swing adsorption under substantially isothermal conditions using an adsorbent comprising a para-selective adsorbent capable of selectively adsorbing para-xylene at a temperature and pressure at which at least 0.01 grams of para-xylene and ethylbenzene may be adsorbed per gram of adsorbent to produce a meta-xylene and ortho-xylene-rich effluent stream comprising a mixture of ortho-xylene and meta-xylene, which contains no more than a total of about 25 mole percent of para-xylene and ethylbenzene based on total C8 aromatics, and a para-xylene-rich effluent stream comprising para-xylene and ethylbenzene, which contains no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics;
(b) sending at least a portion of the para-xylene-rich stream to a crystallization unit and crystallizing said para-xylene-rich stream to produce a para-xylene product stream and a para-xylene-lean mother liquor stream;
(c) sending at least a portion of the meta-xylene and ortho-xylene-rich stream to an isomerization unit and isomerizing said stream to produce an isomerizate comprising an equilibrium mixture of xylenes;
(d) recycling at least a portion of the isomerizate from step (c) to step (a);
(e) sending at least a portion of the para-xylene-lean mother liquor stream from step (b) to an isomerization unit and isomerizing said stream to produce an isomerizate comprising an equilibrium mixture of xylenes; and
(f) recycling at least a portion of the isomerizate from step (e) to step (a).
The invention also relates to the above process of wherein the gaseous mixture comprising meta-xylene, ortho-xylene, para-xylene, and ethylbenzene is contacted with an ethylbenzene conversion catalyst to remove at least a portion of the ethylbenzene prior to being subjected to pressure swing adsorption in step (a).
The invention also relates to the above process wherein at least a portion of the para-xylene-lean mother liquor stream from step (b) is contacted with an ethylbenzene conversion catalyst to remove at least a portion of any ethylbenzene in the para-xylene-lean mother liquor stream and to produce an ethylbenzene-lean effluent which is then recycled to step (a).
The invention also relates to a process for separating and recovering para-xylene from a gaseous mixture comprising C8 aromatic hydrocarbons, the process comprising:
(a) introducing a gaseous mixture comprising meta-xylene, ortho-xylene, para-xylene, and ethylbenzene into a pressure swing adsorption unit and subjecting the mixture to pressure swing adsorption under substantially isothermal conditions using an adsorbent comprising a para-selective adsorbent capable of selectively adsorbing para-xylene at a temperature and pressure at which at least 0.01 grams of para-xylene and ethylbenzene may be adsorbed per gram of adsorbent to produce a meta-xylene and ortho-xylene-rich effluent stream comprising a mixture of ortho-xylene and meta-xylene, which contains no more than a total of about 25 mole percent of para-xylene and ethylbenzene based on total C8 aromatics, and a para-xylene-rich effluent stream comprising para-xylene and ethylbenzene, which contains no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics;
(b) sending at least a portion of the para-xylene-rich stream to a crystallization unit and crystallizing said para-xylene-rich stream to produce a para-xylene product stream and a para-xylene-lean mother liquor stream;
(c) sending at least a portion of the meta-xylene and ortho-xylene-rich stream to an isomerization unit and isomerizing said stream to produce an isomerizate comprising an equilibrium mixture of xylenes;
(d) recycling at least a portion of the isomerizate from step (c) to step (a);
(e) sending at least a portion of the para-xylene-lean mother liquor stream from step (b) to an ethylbenzene conversion unit and contacting it with an ethylbenzene conversion catalyst to remove at least a portion of any ethylbenzene in the para-xylene-lean mother liquor stream and to produce an ethylbenzene-lean effluent; and
(f) combining at least a portion of the ethylbenzene-lean effluent produced in step (e) with the isomerizate from step (c) and recycling the combined effluents to step (a).
The present invention relates to a pressure swing adsorption method for separating para-xylene from a gaseous feed mixture containing meta-xylene and ortho-xylene under substantially isothermal conditions comprising:
(a) adsorbing the mixture onto an adsorbent containing a molecular sieve capable of selectively adsorbing para-xylene at a temperature and pressure at which at least 0.01 grams of para-xylene may be adsorbed per gram of molecular sieve contained in the adsorbent;
(b) producing a first effluent stream containing a mixture of ortho-xylene and meta-xylene, having no more than a total of about 25 mole percent of para-xylene based on total C8 aromatics, preferably less than about 25 mole percent of para-xylene, more preferably no more than about 20 mole percent of para-xylene, more preferably less than about 20 mole percent of para-xylene, more preferably no more than about 15 mole percent of para-xylene, more preferably less than about 15 mole percent of para-xylene, more preferably no more than about 10 mole percent of para-xylene, more preferably less than about 10 mole percent of para-xylene, more preferably no more than about 5 mole percent of para-xylene, more preferably less than about 5 mole percent of para-xylene, more preferably no more than about 3 mole percent of para-xylene, more preferably less than about 3 mole percent of para-xylene, more preferably no more than about 1 mole percent of para-xylene, and most preferably less than about 1 mole percent of para-xylene based on total C8 aromatics;
(c) selectively removing feed from the non-selective void volume;
(d) selectively desorbing para-xylene by decreasing partial pressure of para-xylene;
(e) collecting a stream containing para-xylene and having no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics; preferably less than about 50 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 45 mole percent of meta-xylene and ortho-xylene, more preferably less than about 45 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 40 mole percent of meta-xylene and ortho-xylene, preferably less than about 40 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 35 mole percent of meta-xylene and ortho-xylene, more preferably less than about 35 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 30 mole percent of meta-xylene and ortho-xylene, more preferably less than about 30 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 25 mole percent of meta-xylene and ortho-xylene, more preferably less than about 25 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 20 mole percent of meta-xylene and ortho-xylene, more preferably less than about 20 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 15 mole percent of meta-xylene and ortho-xylene, more preferably less than about 15 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 10 mole percent of para-xylene, more preferably less than about 10 mole percent of meta-xylene and ortho-xylene, more preferably no more than about 5 mole percent of meta-xylene and ortho-xylene, and most preferably less than about 5 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics; and
(f) subjecting the para-xylene-containing stream collected in step (e) to crystallization and recovering high purity para-xylene.
The meta-xylene and ortho-xylene-containing stream from step (b) may be contacted with a catalyst system comprising at least one catalyst which isomerizes meta-xylene and ortho-xylene to give an equilibrium mixture of meta-xylene, ortho-xylene, and para-xylene which may then be sent to the PSA unit for separation.
A practice of the invention involves principally proceeding by repeated cycles comprising in an individual cycle the above steps (a) through (e) to obtain a C8 aromatic stream containing at least about 50 mole percent para-xylene; preferably at least about 55 mole percent para-xylene, more preferably at least about 60 mole percent para-xylene, more preferably at least about 65 mole percent para-xylene, more preferably at least about 70 mole percent para-xylene, more preferably at least about 75 mole percent para-xylene, more preferably at least about 80 mole percent para-xylene, more preferably at least about 85 mole percent para-xylene, more preferably at least about 90 mole percent para-xylene, and more preferably at least about 95 mole percent para-xylene which may then be subjected to crystallization to obtain a purified para-xylene product.
In step (a) of the process of the present invention described above, it is preferable that at least 0.01 g of para-xylene be adsorbed per gram of para-selective adsorbent contained in the adsorbent; more preferable that at least 0.02 g of para-xylene be adsorbed per gram of para-selective adsorbent contained in the adsorbent; and even more preferable that at least 0.03 g of para-xylene be adsorbed per gram of para-selective adsorbent contained in the adsorbent.
Preferably, the first effluent stream mixture of ortho-xylene and meta-xylene produced in the process of the invention, as, for example, in step (b) above, will contain no more than about 25 mole percent of para-xylene based on total C8 aromatics, preferably less than about 25 mole percent of para-xylene, more preferably no more than about 20 mole percent of para-xylene, more preferably less than about 20 mole percent of para-xylene, more preferably no more than about 15 mole percent of para-xylene, more preferably less than about 15 mole percent of para-xylene, more preferably no more than about 10 mole percent of para-xylene, more preferably less than about 10 mole percent of para-xylene, more preferably no more than about 5 mole percent of para-xylene, more preferably less than about 5 mole percent of para-xylene, more preferably no more than about 3 mole percent of para-xylene, more preferably less than about 3 mole percent of para-xylene, and still more preferably no more than about 1 mole percent of para-xylene, and even more preferably less than about 1 mole percent of para-xylene.
Preferably, the para-xylene-containing stream collected in the process of the invention, as, for example, in step (e) above, will contain no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics, preferably less than a total of about 50 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 45 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 45 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 40 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 40 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 30 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 30 mole percent of meta-xylene and ortho-xylene, preferably no more than a total of about 25 mole percent of meta-xylene and ortho-xylene; preferably less than a total of about 25 mole percent of meta-xylene and ortho-xylene; more preferably no more than a total of about 20 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 20 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 15 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 15 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 10 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 10 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 5 mole percent of meta-xylene and ortho-xylene, and most preferably less than a total of about 5 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics.
In the most preferred embodiments of the invention, the effluent product stream containing para-xylene, or para-xylene and ethylbenzene, will be substantially free of meta-xylene and ortho-xylene, and the effluent product stream containing meta-xylene and ortho-xylene will be substantially free of para-xylene, or substantially free of para-xylene and ethylbenzene.
The molecular sieve is preferably a para-selective, non-acidic medium pore molecular sieve. Preferably, the molecular sieve comprises silicalite, and more preferably, the molecular sieve comprises orthorhombic crystals of silicalite having an average minimum dimension of at least about 0.2 xcexcm.
In one embodiment of the invention, the adsorbent comprises a para-selective, adsorbent and a binder. The binder is preferably selected from the group consisting of clay, alumina, silica, titania, zirconia, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia, silica-magnesia-zirconia, and aluminum phosphate.
A para-selective adsorbent is an adsorbent that, when subjected to an equal molar mixture of C8 aromatics at 50xc2x0 C., adsorbs pX and EB preferentially over mX and oX, such that the total pX and EB in the adsorbate is at least about 75% relative to the total C8 aromatics.
A preferred para-selective adsorbent, when subjected to an equal molar mixture of C8 aromatics at 50xc2x0 C., will adsorb pX and EB preferentially over mX and oX, such that the total pX and EB in the adsorbate is greater than about 75% relative to the total C8 aromatics.
A more preferred para-selective adsorbent, when subjected to an equal molar mixture of C8 aromatics at 50xc2x0 C., will adsorb pX and EB preferentially over mX and oX, such that the total pX and EB in the adsorbate is at least about 80% relative to the total C8 aromatics, even more preferably, at least about 85% relative to the total C8 aromatics, still more preferably, at least about 90% relative to the total C8 aromatics; and yet more preferably, at least about 95% relative to the total C8 aromatics; and most preferably, at least about 97% relative to the total C8 aromatics.
In the present invention the pressure swing adsorption operating temperature is preferably at least about 350xc2x0 F., preferably about 350xc2x0 F. to about 750xc2x0 F., more preferably from about 400xc2x0 F. to about 750xc2x0 F. more preferably from about 450xc2x0 F. to about 750xc2x0 F. and the operating pressure is at least about 30 psia (206 kPa), preferably about 50 psia (345 kPa) to about 400 psia, more preferably from about 100 psia to about 400 psia (from about 206 kPa to about 2760 kPa).
In the process of the present invention, the para-xylene or para-xylene and ethylbenzene rich stream from the pressure swing adsorption unit is fed to a fractional crystallization unit. Crystallization is conducted by passing the mixture into a crystallizer maintained at a temperature sufficient to induce crystallization of pX, generally between about 40xc2x0 F. and xe2x88x92130xc2x0 F. The lower the temperature, the more pX that can be removed from the system. An advantage of the process of the invention is that a significant portion of the mX and oX are removed by the PSA unit. As a result, the temperature at which pX can be crystallized without co-crystallization of impurities is lowered substantially, allowing a higher percentage of pX product to be removed from the system per pass. Crystallization can be done in one stage or multiple stages. When the para-xylene concentration in the stream from the pressure swing adsorption unit is sufficiently, high, crystallization of para-xylene may be conducted at temperatures of about 10xc2x0 F. to abut 55xc2x0 F.
In a preferred embodiment of the invention, the para-xylene-enriched stream from the PSA unit is crystallized in a first crystallizer at a temperature of about 10xc2x0 F. to about 55xc2x0 F.; an effluent comprising para-xylene crystals in a mother liquor is recovered, and the para-xylene crystals are separated from the mother liquor in a first separation unit, washed with liquid para-xylene, and completely melted to give a liquid para-xylene product of high purity. The filtrate from this first crystallization is subjected to fractional crystallization to recover additional para-xylene product.
The present invention additionally relates to a pressure swing adsorption method to separate para-xylene and ethylbenzene from a gaseous feed mixture containing meta-xylene and ortho-xylene under substantially isothermal conditions comprising:
(a) adsorbing the mixture onto an adsorbent containing a molecular sieve capable of selectively sorbing para-xylene and ethylbenzene at a temperature and pressure at which at least 0.01 grams of para-xylene and ethylbenzene may be adsorbed per gram of molecular sieve;
(b) producing a first effluent stream containing a mixture of ortho-xylene and meta-xylene having no more than a total of about 25 mole percent of para-xylene and ethylbenzene based on total C8 aromatics, preferably less than about 25 mole percent of para-xylene and ethylbenzene, more preferably no more than about 20 mole percent of para-xylene and ethylbenzene, more preferably less than about 20 mole percent of para-xylene and ethylbenzene, more preferably no more than about 15 mole percent of para-xylene and ethylbenzene, more preferably less than about 15 mole percent of para-xylene and ethylbenzene, more preferably no more than about 10 mole percent of para-xylene and ethylbenzene, more preferably less than about 10 mole percent of para-xylene and ethylbenzene, more preferably no more than about 5 mole percent of para-xylene and ethylbenzene, more preferably less than about 5 mole percent of para-xylene and ethylbenzene, more preferably no more than about 3 mole percent of para-xylene and ethylbenzene, more preferably less than about 3 mole percent of para-xylene and ethylbenzene, more preferably no more than about 1 mole percent of para-xylene and ethylbenzene, and most preferably less than about 1 mole percent of para-xylene and ethylbenzene based on total C8 aromatics;
(c) selectively removing feed from the non-selective void volume;
(d) selectively desorbing para-xylene and ethylbenzene by decreasing partial pressure of para-xylene and ethylbenzene; and
(e) collecting a stream containing para-xylene and ethylbenzene and having no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics.
(f) subjecting the para-xylene and ethylbenzene-containing stream collected in step (e) to crystallization and recovering high purity para-xylene.
In one embodiment of the present invention, the effluent stream containing a mixture of ortho-xylene and meta-xylene may be contacted with a catalyst system comprising at least one catalyst which isomerizes meta-xylene and ortho-xylene to give an equilibrium mixture of meta-xylene, ortho-xylene, and para-xylene which may then be sent to the PSA unit for separation. When the gaseous feed stream additionally contains ethylbenzene, the meta-xylene and ortho-xylene-containing effluent stream may be contacted with a catalyst system comprising at least one catalyst which converts at least a portion of any ethylbenzene in the stream to products that can be separated from the xylenes by distillation and which isomerizes meta-xylene and ortho-xylene to give an equilibrium mixture of meta-xylene, ortho-xylene, and para-xylene which may then be sent to the PSA unit for separation.
A para-xylene-lean reject stream from the crystallization unit which comprises C8 aromatics may be sent to a catalyst reactor, where the xylenes are isomerized to equilibrium and where at least a portion of the ethylbenzene is converted to products which can be separated by fractional distillation from the C8 aromatics. The catalyst or combination of catalysts in the reactor can be any that are suitable for xylene isomerization and ethylbenzene conversion, as known to those skilled in the art. Examples of such catalysts are described in EP 138,617, U.S. Pat. No. 5,001,296, U.S. Re. 31,782, U.S. Pat. Nos. 4,098,836 and 4,899,011 incorporated herein by reference in their entireties. Suitable isomerization conditions include a temperature of about 250xc2x0 C. to about 500xc2x0 C., preferably about 340xc2x0 C. to about 430xc2x0 C., a pressure of about atmospheric to about 400 psig, preferably in the range of about 100 psig to about 300 psig, a hydrogen to hydrocarbon mole ratio of about 0.5:1 to about 10:1, and a liquid weight hourly space velocity of about 0.5 to about 100 hrxe2x88x921. The para-xylene-lean reject stream may be combined with the mX/oX-rich effluent stream from the PSA prior to sending it to the isomerization reactor.
An additional catalyst reactor may be used to pretreat the C8 aromatic feed to convert at least a portion of the ethylbenzene to products which can be separated by fractional distillation from the C8 aromatics prior to sending the C8 aromatic feedstream to the PSA unit.
In an alternate embodiment, an additional catalyst reactor may be used to treat the para-xylene-lean reject stream from the crystallizer to convert at least a portion of the ethylbenzene in it to xylenes or products which can be separated by fractional distillation from the C8 aromatics prior to sending the C8 aromatic feedstream to the PSA unit.
The catalyst system in the additional catalyst reactor used to convert ethylbenzene can be any catalyst system suitable for ethylbenzene dealkylation, hydrodeethylation or hydroisomerization. Examples of catalyst systems for dealkylation are disclosed in Re. 31,782 and U.S. Pat. No. 4,908,342, incorporated herein by reference in their entireties. Examples of catalyst systems for hydrodeethylation are disclosed in U.S. Pat. Nos. 4,899,011 and 5,367,099 incorporated herein by reference in their entireties. Examples of catalyst systems for hydroisomerization are disclosed in U.S. Pat. Nos. 5,028,573, 6,150,292 and 5,908,967 incorporated herein by reference in their entireties.
A practice of the invention involves principally proceeding by repeated cycles comprising in an individual cycle the above steps (a) through (e) to produce an effluent stream comprising para-xylene and ethylbenzene containing no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics from which para-xylene is separated and purified via crystallization, and another effluent stream comprising meta-xylene and ortho-xylene containing no more than a total of about 20 mole percent of para-xylene based on total C8 aromatics which is contacted with an isomerization catalyst system to produce an equilibrium mixture of meta-xylene, ortho-xylene, and para-xylene.
In step (a) of the process of the present invention described above, it is preferable that at least 0.01 g of para-xylene and ethylbenzene be adsorbed per gram of molecular sieve contained in the adsorbent; more preferable that at least 0.02 g of para-xylene and ethylbenzene be adsorbed per gram of molecular sieve contained in the adsorbent; still more preferable that at least 0.03 g of para-xylene and ethylbenzene be adsorbed per gram of molecular sieve contained in the adsorbent.
The present invention also relates to a process for separating a mixture of organic compounds having normal boiling points in a temperature range from about 80xc2x0 C. to about 160xc2x0 C., which process comprises:
(a) providing an adsorbent bed comprising a molecular sieve which exhibits capacity to selectively adsorb and desorb para-xylene and ethylbenzene under substantially isothermal conditions of temperature at operating pressure, disposed in a vessel having at least one inlet and at least one outlet such that gas entering an inlet passes through the adsorbent bed to an outlet, and containing a purge gas substantially free of C8 aromatic compounds;
(b) flowing a gaseous feed mixture comprising xylenes and ethylbenzene into the bed through one or more of the vessel inlets, and collecting an effluent from one or more of the outlets comprising purge gas substantially free of C8 aromatic compounds while selectively adsorbing para-xylene and ethylbenzene from the gaseous mixture under substantially isothermal conditions in the bed;
(c) continuing the flow of gaseous feed and collecting from one or more of the outlets and segregating a second effluent comprising m-xylene and o-xylene having no more than about 25 mole percent of p-xylene and ethylbenzene based on total C8 aromatics;
(d) stopping the feed mixture flowing into the bed through one or more inlets just prior to breakthrough (i.e., the adsorption front is close to the exit end of the adsorbent column), and flowing purge gas preferably in a direction counter to the direction of the C8 aromatic feed, while maintaining substantially isothermal conditions in the bed, and collecting from one or more of the outlets an effluent gaseous mixture of C8 aromatic feed until effluent at the outlet contains no more than about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics;
(e) continuing the flow of purge gas and collecting from one or more of the outlets and segregating an effluent comprising ethylbenzene and p-xylene which contains no more than about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics;
(f) subjecting the para-xylene-containing stream collected in step (e) to crystallization and recovering high purity para-xylene; and
(h) optionally, contacting the meta-xylene and ortho-xylene-containing stream from step (c) with a catalyst system, comprising at least one catalyst, which converts at least a portion of the ethylbenzene in the stream to products that can be separated from the xylenes by distillation and which isomerizes meta-xylene and ortho-xylene to produce an equilibrium mixture of meta-xylene, ortho-xylene, and para-xylene
In a preferred embodiment of the above process, in step (d) the effluent gaseous mixture of C8 aromatic feed will be collected until the effluent at the outlet is substantially free of meta-xylene and ortho-xylene.
A practice of the invention involves principally proceeding by repeated cycles comprising in an individual cycle the above steps (a) through (e) to produce an effluent stream comprising para-xylene and ethylbenzene containing no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics from which para-xylene is separated and purified via crystallization, and another effluent stream comprising meta-xylene and ortho-xylene containing no more than a total of about 25 mole percent of para-xylene which is contacted with a catalyst system, comprising at least one catalyst, which converts at least a portion of the ethylbenzene in the stream to products that can be separated from the xylenes by distillation and which isomerizes meta-xylene and ortho-xylene to give an equilibrium mixture of meta-xylene, ortho-xylene, and para-xylene to produce an equilibrium mixture of meta-xylene, ortho-xylene, and para-xylene.
In a preferred embodiment of the process, the flow of the purge gas is counter current to the flow of the gaseous feed mixture.
In one embodiment of the process, steps (b) through (e) are repeated with a cycle time of from about 2 minutes to about 200 minutes, preferably with a cycle time of from about 3 minutes to about 50 minutes, more preferably with a cycle time of from about 3 minutes to about 30 minutes.
In an embodiment of the process at least a portion of the effluent gaseous mixture collected in step (d) is admixed with the gaseous feed mixture in subsequent cycles.
In another embodiment of the process, the purge gas comprises hydrogen, and steps (b) through (e) are repeated with a cycle time of from about 3 minutes to about 30 minutes under substantially isothermal conditions at a temperature of about 350xc2x0 F. to about 750xc2x0 F. and at constant operating pressure at a pressure of at least about 30 psia to about 400 psia.
An additional embodiment of the invention comprises a process for separating a mixture of ethylbenzene and the isomers of xylene, which process comprises:
(a) providing an adsorbent bed comprising a molecular sieve which exhibits capacity to selectively adsorb and desorb para-xylene and ethylbenzene under substantially isothermal conditions at operating pressure, disposed in a vessel having at least one inlet and at least one outlet such that gas entering an inlet passes through the particulate bed to an outlet and pressurizing the vessel with a mixture comprising meta-xylene and ortho-xylene to a preselected pressure for adsorption;
(b) flowing a gaseous feed mixture comprising xylene isomers and ethylbenzene into the adsorbent bed through one or more inlets and displacing the meta-xylene and ortho-xylene in the vessel while selectively adsorbing ethylbenzene and para-xylene from the gaseous feed mixture under substantially isothermal conditions in the adsorbent bed;
(c) collecting from one or more of the outlets a first effluent product comprising meta-xylene and ortho-xylene which contains no more than a total of about 25 mole percent of ethylbenzene and para-xylene based on total C8 aromatics while maintaining substantially isothermal conditions in the adsorbent bed and the flow of feed at the pressure for adsorption;
(d) replacing the feed mixture flowing into the bed though one or more inlets with a purge gas comprising para-xylene and ethylbenzene substantially free of meta-xylene and ortho-xylene while maintaining the pressure for adsorption and substantially isothermal conditions in the bed, and collecting from one or more of the outlets a gaseous mixture comprising feed;
(e) reducing the pressure to desorb ethylbenzene and para-xylene while maintaining substantially isothermal conditions in the bed; and
(f) collecting a second effluent product comprising ethylbenzene and para-xylene which contains no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics.
In a preferred embodiment of the above process:
(a) the flow of said para-xylene and ethylbenzene purge gas is countercurrent to the flow of the gaseous feed mixture;
(b) the para-xylene and ethylbenzene effluent flow during depressurization is countercurrent to the flow of the gaseous feed mixture; and
(c) the flow of meta-xylene and ortho-xylene to pressurize the vessel is countercurrent to the feed gas flow.
A further embodiment of the invention comprises a process for separating a mixture of ethylbenzene and the isomers of xylene, which process comprises:
(a) providing at least two adsorbent beds containing a molecular sieve which exhibits capacity to selectively adsorb and desorb para-xylene and ethylbenzene under substantially isothermal conditions at operating pressure, disposed in sequentially connected or interconnected vessels, each having at least one inlet and at least one outlet such that gas entering an inlet passes through the particulate bed to an outlet, and pressurizing a first vessel with a mixture comprising meta-xylene and ortho-xylene to a preselected pressure for adsorption;
(b) flowing a gaseous feed mixture comprising xylene isomers and ethylbenzene into the adsorbent bed in the first vessel though one or more inlets and displacing the meta-xylene and ortho-xylene in the vessel while selectively adsorbing ethylbenzene and para-xylene from the gaseous feed mixture under substantially isothermal conditions in the adsorbent bed;
(c) collecting from one or more of the outlets a first effluent product comprising meta-xylene and ortho-xylene which contains no more than a total of about 25 mole percent of ethylbenzene and para-xylene based on total C8 aromatics while maintaining substantially isothermal conditions in the adsorbent bed and the flow of feed at the pressure for adsorption;
(d) stopping the flow of feed and reducing the pressure in the first vessel sufficiently to permit removal of at least a portion of the feed from non-selective voids while maintaining substantially isothermal conditions in the bed by equalizing the pressure in the first vessel with the pressure in the second vessel which is at a lower pressure;
(e) further reducing the pressure in the first vessel to desorb ethylbenzene and para-xylene while maintaining substantially isothermal conditions in the bed; and
(f) collecting a second effluent product comprising ethylbenzene and para-xylene which contains no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics.
In the above process, following step (f), a purge gas comprising meta-xylene and ortho-xylene can be added to the first vessel to displace para-xylene and ethylbenzene in the non-selective voids, and an effluent comprising the para-xylene and ethylbenzene is collected.
Another embodiment of the present invention comprises a process for separating a mixture of ethylbenzene and the isomers of xylene, which process comprises:
(a) providing an adsorbent bed comprising a molecular sieve which exhibits capacity to selectively adsorb and desorb para-xylene and ethylbenzene under substantially isothermal conditions at operating pressure, disposed in a vessel having at least one inlet and at least one outlet such that gas entering an inlet passes through the particulate bed to an outlet and pressurizing the vessel with a mixture of substantially meta-xylene and ortho-xylene to a preselected pressure for adsorption;
(b) flowing a gaseous feed mixture comprising xylene isomers and ethylbenzene into the adsorbent bed though one or more inlets and displacing the meta-xylene and ortho-xylene in the vessel while selectively adsorbing ethylbenzene and para-xylene from the gaseous feed mixture under substantially isothermal conditions in the adsorbent bed;
(c) collecting from one or more of the outlets a first effluent product comprising meta-xylene and ortho-xylene which contains no more than a total of about 25 mole percent of ethylbenzene and para-xylene based on total C8 aromatics while maintaining substantially isothermal conditions in the adsorbent bed and the flow of feed at the pressure for adsorption;
(d) stopping the flow of feed and reducing operating pressure to a pressure at which para-xylene and ethylbenzene desorb while maintaining substantially isothermal conditions in the bed; and
(e) collecting a second effluent product comprising ethylbenzene and para-xylene which contains no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics.
In the above embodiment, preferably, following step (e), a purge gas comprising meta-xylene and ortho-xylene is added to the first vessel to displace para-xylene and ethylbenzene in the non-selective voids, and an effluent comprising the para-xylene and ethylbenzene is collected.
In the embodiments of the pressure swing adsorption process of the present invention described above, it is preferred that the first effluent stream mixture of ortho-xylene and meta-xylene produced in the process of the invention will contain no more than about 25 mole percent of para-xylene based on total C8 aromatics, preferably less than about 25 mole percent of para-xylene, more preferably no more than about 20 mole percent of para-xylene, more preferably less than about 20 mole percent of para-xylene, more preferably no more than about 15 mole percent of para-xylene, more preferably less than about 15 mole percent of para-xylene, more preferably no more than about 10 mole percent of para-xylene, more preferably less than about 10 mole percent of para-xylene, more preferably no more than about 5 mole percent of para-xylene, more preferably less than about 5 mole percent of para-xylene, more preferably no more than about 3 mole percent of para-xylene, more preferably less than about 3 mole percent of para-xylene, and still more preferably no more than about 1 mole percent of para-xylene.
In the embodiments of the pressure swing adsorption process of the present invention described above wherein the first effluent mX/oX stream contains both para-xylene and ethylbenzene, it is preferred that the first effluent stream mixture of ortho-xylene and meta-xylene produced in the process of the invention will contain no more than about 25 mole percent of para-xylene and ethylbenzene based on total C8 aromatics, preferably less than about 25 mole percent of para-xylene and ethylbenzene, more preferably no more than about 20 mole percent of para-xylene and ethylbenzene, more preferably less than about 20 mole percent of para-xylene and ethylbenzene, more preferably no more than about 15 mole percent of para-xylene and ethylbenzene, more preferably less than about 15 mole percent of para-xylene and ethylbenzene, more preferably no more than about 10 mole percent of para-xylene and ethylbenzene, more preferably less than about 10 mole percent of para-xylene and ethylbenzene, more preferably no more than about 5 mole percent of para-xylene and ethylbenzene, more preferably less than about 5 mole percent of para-xylene and ethylbenzene, more preferably no more than about 3 mole percent of para-xylene and ethylbenzene, more preferably less than about 3 mole percent of para-xylene and ethylbenzene, and still more preferably no more than about 3 mole percent of para-xylene and ethylbenzene.
In the embodiments of the pressure swing adsorption process of the present invention described above, it is preferred that the para-xylene-containing stream collected in the process of the invention will contain no more than a total of about 50 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics, preferably less than a total of about 50 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 45 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 45 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 40 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 40 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 30 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 30 mole percent of meta-xylene and ortho-xylene, preferably no more than a total of about 25 mole percent of meta-xylene and ortho-xylene; preferably less than a total of about 25 mole percent of meta-xylene and ortho-xylene; more preferably no more than a total of about 20 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 20 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 15 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 15 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 10 mole percent of meta-xylene and ortho-xylene, preferably less than a total of about 10 mole percent of meta-xylene and ortho-xylene, more preferably no more than a total of about 5 mole percent of meta-xylene and ortho-xylene, and most preferably less than a total of about 5 mole percent of meta-xylene and ortho-xylene based on total C8 aromatics.
In the most preferred embodiments of the pressure swing adsorption process of the present invention, the effluent product stream containing para-xylene, or para-xylene and ethylbenzene, will be substantially free of meta-xylene and ortho-xylene, and the effluent product stream containing meta-xylene and ortho-xylene will be substantially free of para-xylene, or substantially free of para-xylene and ethylbenzene.
A purge gas substantially free of C8 aromatic compounds will contain no more than about 10 wt %, and preferably less than about 5 wt %, and most preferably less than about 2 wt % of C8 aromatic compounds.
A fraction or stream substantially free of p-xylene and ethylbenzene will contain no more than a total of about 5 mole percent of p-xylene and ethylbenzene based on total C8 aromatics.
A fraction or stream substantially free of para-xylene will contain no more than about 5 mole percent of para-xylene based on total C8 aromatics. Preferably such a fraction will contain no more than about 1 mole percent of para-xylene based on total C8 aromatics.
For those process steps conducted at constant pressure, those skilled in the art will recognize that during operation there may be slight variations in pressure due to pressure drops across the system or changes in flows; however the pressure will remain substantially constant.
A fraction or stream substantially free of m-xylene and o-xylene will contain no more than a total of about 25 mole percent of m-xylene and o-xylene based on total C8 aromatics. Preferably such a stream will contain no more than about 20 mole percent, more preferably no more than about 15 mole percent; still more preferably no more than about 10 mole percent; and most preferably no more than about 5 mole percent of m-xylene and o-xylene based on total C8 aromatics.
The present invention also relates to a method of pressure swing adsorption which includes a plurality of steps and which provides recovery from a mixture comprising C8 aromatics of a product stream of p-xylene or p-xylene and ethylbenzene which is substantially free of m-xylene and o-xylene as well as a product stream of meta-xylene and ortho-xylene which is substantially free of p-xylene and ethylbenzene. The present invention provides a pressure swing adsorption process whereby there can be obtained from a feed comprising C8 aromatics a high yield of a high purity product stream of p-xylene and ethylbenzene and also a high yield of a high purity product stream of m-xylene and o-xylene.